The Electric Atmosphere: Plasma Is Next NASA Science Target

Our day-to-day lives exist in what physicists would call an electrically
neutral environment. Desks, books, chairs and bodies don't generally
carry electricity and they don't stick to magnets. But life on Earth is
substantially different from, well, almost everywhere else. Beyond
Earth's protective atmosphere and extending all the way through
interplanetary space, electrified particles dominate the scene. Indeed,
99% of the universe is made of this electrified gas, known as plasma.

Two giant donuts of this plasma surround Earth, trapped within a region
known as the Van Allen Radiation Belts. The belts lie close to Earth,
sandwiched between satellites in geostationary orbit above and
satellites in low Earth orbit (LEO) are generally below the belts. A new
NASA mission called the Radiation Belt Storm Probes (RBSP), due to
launch in August 2012, will improve our understanding of what makes
plasma move in and out of these electrified belts wrapped around our
planet.

"We discovered the radiation belts in observations from the very first
spacecraft, Explorer 1, in 1958" says David Sibeck, a space scientist at
NASA's Goddard Space Flight Center in Greenbelt, Md., and the mission
scientist for RBSP. "Characterizing these belts filled with dangerous
particles was a great success of the early space age, but those
observations led to as many questions as answers. These are fascinating
science questions, but also practical questions, since we need to
protect satellites from the radiation in the belts."

The inner radiation belt stays largely stable, but the number of
particles in the outer one can swell 100 times or more, easily
encompassing a horde of communications satellites and research
instruments orbiting Earth. Figuring out what drives these changes in
the belts, requires understanding what drives the plasma.

› Download video
This visualization relies on data from the SAMPEX mission, which
observed particles in the Radiation Belts during a large solar storm in
October 2003. The movie clearly shows just how much the outer belt can
swell in extreme conditions.
Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

Plasmas seethe with complex movement. They generally flow along a
skeletal structure made of invisible magnetic field lines, while
simultaneously creating more magnetic fields as they move. Teasing out
the rules that govern such a foreign environment – one that can only be
studied from afar – lies at the heart of understanding a range of events
that make up space weather, from giant explosions on the sun to
potentially damaging high energy particles in near-Earth environs.

To distinguish between a host of theories developed over the years on
plasma movement in those near-Earth environs, RBSP scientists have
designed a suite of instruments to answer three broad questions. Where
do the extra energy and particles come from? Where do they disappear to,
and what sends them on their way? How do these changes affect the rest
of Earth's magnetic environment, the magnetosphere? In addition to its
broad range of instruments, the RBSP mission will make use of two
spacecraft in order to better map out the full spatial dimensions of a
particular event and how it changes over time.

Scientists want to understand not only the origins of electrified
particles – possibly from the solar wind constantly streaming off the
sun; possibly from an area of Earth's own outer atmosphere, the
ionosphere – but also what mechanisms gives the particles their extreme
speed and energy.

"We know examples where a storm of incoming particles from the sun can
cause the two belts to swell so much that they merge and appear to form a
single belt," says Shri Kanekal, RBSP's deputy project scientist at
Goddard. "Then there are other examples where a large storm from the sun
didn't affect the belts at all, and even cases where the belts shrank.
Since the effects can be so different, there is a joke within the
community that 'If you've seen one storm . . . You've seen one storm.'
We need to figure out what causes the differences."

There are two broad theories on how the particles get energy: from
radial transport or in situ. In radial transport, particles move
perpendicular to the magnetic fields within the belts from areas of low
magnetic strength far from Earth to areas of high magnetic strength
nearer Earth. The laws of physics dictate that particle energies
correlate to the strength of the magnetic field, increasing as they move
towards Earth. The in situ theory posits that electromagnetic waves
buffet the particles -- much like regular pushes on a swing --
successively raising their speed (and energy).

As for how the particles leave the belts, scientists again agree on two
broad possibilities: particles go up, or they go down. Perhaps they
travel down magnetic field lines toward Earth, out of the belts into the
ionosphere, where they stay part of Earth's magnetic system with the
potential to return to the belts at some point. Or they are transported
up and out, on a one-way trip to leave the magnetosphere forever and
enter interplanetary space.

› View larger
An artist's rendition of what the two Radiation Belt Storm Probe
spacecraft will look like in space. Credit: NASA/Goddard Space Flight
Center
"In reality, the final answers may well be a combination of the basic
possibilities," says Sibeck. "There may be, and probably are, multiple
processes at multiple scales at multiple locations. So RBSP will perform
very broad measurements and observe numerous attributes of waves and
particles to see how each event influences others."

To distinguish between the wide array of potential theories – not to
mention combinations thereof – the instruments on RBSP will be equipped
to measure a wide spectrum of information. RBSP will measure a host of
different particles, including hydrogen, helium and oxygen, as well as
measure magnetic fields and electric fields throughout the belts, both
of which can guide the movement of these particles.

RBSP will also measure a wide range of energies from the coldest
particles in the ionosphere to the most energetic, most dangerous
particles. Information about how the radiation belts swell and shrink
will help improve models of Earth's magnetosphere as a whole.

"Particles from the radiation belts can penetrate into spacecraft and
disrupt electronics, short circuits or upset memory on computers," says
Sibeck. "The particles are also dangerous to astronauts traveling
through the region. We need models to help predict hazardous events in
the belts and right now we are aren’t very good at that. RBSP will help
solve that problem."

While the most immediate practical need for studying the radiation belts
is to understand the space weather system near Earth and to protect
humans and precious electronics in space from geomagnetic storms, there
is another reason scientists are interested in this area. It is the
closest place to study the material, plasma, that pervades the entire
universe. Understanding this environment so foreign to our own is
crucial to understanding the make up of every star and galaxy in outer
space.

The Johns Hopkins University Applied Physics Laboratory (APL) built and
will operate the twin RBSP spacecraft for NASA’s Living With a Star
program, which is managed by Goddard Space Flight Center for NASA’s
Science Mission Directorate.